1.4 Thermal energy transfer by thermal radiation (electromagnetic infrared radiation)

Thermal (heat) radiation is emitted by all materials, gases, liquids or solids and the hotter the material the more strongly it gives out heat radiation which is called infrared radiation (IR).

• (a) All objects continuously emit and absorb infrared radiation from their surface, whatever their temperature - increasing or decreasing their thermal energy stores.

  • Infrared radiation is a form of electromagnetic radiation and can travel through a vacuum.

• (b) The higher the temperature (the hotter) an object is, the more infrared radiation it radiates in a given time, the higher the temperature of the material, the more intense is the infrared radiation.

  • An object that is hotter (higher temperature) than its surroundings will emit more radiation than it absorbs and an object that is cooler than its surroundings will absorb more radiation than it emits.

  • You notice this effect in bright sunlight by feeling the warmth on your hand or standing near a fire.

  • When an object cools down to the same temperature as its surroundings emitted infrared radiation equals the absorbed heat radiation.

• (c) Dark, matt surfaces are good absorbers and good emitters of infrared radiation eg rough black surfaces.

  • Solar panels for hot water comprise of pipes carrying water to be heated set under a black surface to efficiently absorb the infrared radiation from the Sun. You can even just use matt black painted water pipes. You may even have a silvered surface under the pipes so more infrared ins reflected onto the black surface rather than becoming waste heat radiation. The pipes are made of copper which allows efficient conduction of the surface heat energy to the incoming cold water., so the hot water can be used as part of the households domestic heating or washing etc.

• (d) Light, shiny smooth surfaces are poor absorbers and poor emitters of infrared radiation eg white gloss paint, silver surface used in vacuum flask.

  • See 'thermos flask' in part 1.4 for minimising thermal radiation energy transfer.

• (e) Light, shiny surfaces are good reflectors of infrared radiation, this maybe to keep heat in to keep things warm or to minimise heat radiation in to keep things cool eg a vacuum flask.

  • See 'thermos flask' in part 1.4 for minimising thermal radiation energy transfer.

• (f) Car headlamp

(e) Another domestic case of infrared radiation! Unlike 'modern' LED bulbs, 'old fashioned' filament bulbs emit quite a bit of IR heat radiation. You can detect this with a frosty car where the central portion of the ice melts first on the transparent headlamp cover. Filament bulbs only convert ~10% of the electrical energy into visible light energy, most of the rest is converted into infrared radiation. The ice on the headlamp cover absorbs infrared equivalent to the latent heat of fusion (melting) and changes the ice to liquid water.

Energy store changes: The chemical energy store of the battery decreases as it is converted into electrical energy. The electrical energy increases the thermal energy store of the metal filament of the bulb. The thermal energy store of the filament decreases as it emits visible and infrared EM radiation. The absorbed EM radiation increases the thermal energy store of the headlamp cover and ice - causing the latter to melt. Eventually all the energy involved from the battery increases the thermal energy store of the surroundings.

A 'green' note: If there is, and its happening now in the UK and other countries, a change from very inefficient filament light bulbs to very efficient low energy LED light bulbs, there will be quite a reduction in the domestic demand for electricity. This reduced demand will help, on closure fossil fuel power stations, reduce CO₂ emissions, reducing the greenhouse effect, and allow renewable energy resources to take over more of our electricity generation.

(f) The electrical resistance elements of a cooker ring or a toaster become hot enough to emit a strong beam of infrared radiation to heat the contents of a pan or grill the toast. Electrical energy is converted into heat energy which increases the thermal energy store of the elements and then increases the thermal energy store of the pan and contents or bread being toasted.

See also More on methods of reducing heat transfer

Look up Leslie cube experiment via

 The absorption and emission of radiation by materials - temperature & surface factors

Notes on thermal energy transfer by conduction, convection, radiation